The present invention relates to a blown film whose optical characteristics, tear strength and rigidity are well-balanced.
A blown film is employed widely for packaging since it enables a film production at a high productivity with simple equipment. Such a blown film should have, for example, an optical property (transparency or gloss), tear strength and rigidity. In response to a trend of demanding a thinner film, a film having excellent tear resistance and rigidity is required.
While a blown film made of a low density polyethylene (LDPE) by a high pressure radical polymerization process has excellent optical characteristics, it has a poor tear resistance. On the other hand, while a blown film made of a linear low density polyethylene (LLDPE) which is a copolymer of ethylene and an a-olefin has excellent tear strength, it has poor optical characteristics. The optical characteristics are usually improved by blending 10 to 30% of LDPE with LLDPE, but the tear strength of the film thus obtained is deteriorated.
The rigidity of a film made of LLDPE or LDPE is related with the density, and a higher density usually gives a higher rigidity, but is accompanied with a deterioration of the optical characteristics and a reduction in the tear strength, thus to obtain a satisfactory rigidity of a film simultaneously with satisfactory optical characteristics and tear strength is difficult.
An objective of the present invention is to provide a blown film which is excellent in terms of the optical characteristics, the tear strength and the rigidity, as well as a method for producing the same.
The present invention is directed to a blown film whose surface morphology is a spherulite structure in which spherulites having diameters of about 3 xcexcm or more exist and whose surface average roughness Ra is about 30 nm or less, as well as a method for producing the same.
This invention is further detailed below.